Persistence of an endangered native duck, feral mallards, and multiple hybrid swarms across the main Hawaiian Islands

The endangered, endemic Hawaiian Duck is thought to be threatened with genetic extinction through ongoing hybridization with an introduced congener, the feral mallard. We investigated spatial and temporal variation in hybrid prevalence in populations throughout the main Hawaiian Islands, using genomic data to characterize population structure of Hawaiian Duck, quantify the extent of hybridization, and compare hybrid proportions over time. To accomplish this, we genotyped 3,308 double-digest restriction-site-associated DNA (ddRAD) loci in 425 putative Hawaiian Duck, mallards, and hybrids from populations across the main Hawaiian Islands. We found that despite a population decline in the last century, Hawaiian Duck genetic diversity is high. There were few hybrids on the island of Kaua?i, home to the largest population of Hawaiian Duck. By contrast, we report that sampled populations outside of Kaua?i can now be characterized as hybrid swarms, in that all individuals sampled were of mixed Hawaiian Duck × mallard ancestry. Further, there is some evidence that these swarms are stable over time. These findings demonstrate spatial variation in the extent and consequences of interspecific hybridization, and highlight how islands or island-like systems with small population sizes may be especially prone to genetic extinction when met with a congener that is not reproductively isolated.